Dual-Mode Operation and Transition in Axisymmetric Scramjets

Abstract

Dual-mode operation and mode transition are experimentally investigated in optical axisymmetric scramjets in Mach 4.5 flows. The axisymmetric scramjet emphasizes generic combustion and flameholding dynamics without the corner boundary-layer effects that can distort flame propagation in rectangular geometries. Flow and combustion dynamics are characterized by pressure transducers distributed along the combustor and heat flux measurements at the combustor exit. Flame structures are resolved by instantaneous OH planar laser-induced fluorescence and high-speed flame chemiluminescence imaging. The initiation of combustion mode transition depends on the amount of heat addition in relation to the area relief along the flowpath. Mixing enhancement by a cavity enables significant combustion heat addition into the core flow such that thermal choking can be attained in the diverging combustor. Thermal choking initiates at the pressure inflection point where the effects of combustion heat loading and area relief are comparable in the diverging combustor. A steady ramjet combustion is established in flow regions upstream of the diverging combustor after scram-to-ram mode transition and the flow reaccelerates to supersonic speed after the cavity ramp.